Twenty years after the Chernobyl nuclear accident, debate still rages over the health impact from the nuclear fallout it deposited. A United Nations-sponsored report last year estimated the number of people that might suffer long-term physical consequences such as cancer, but critics say it greatly understates the potential.

There is no disagreement that the explosion of a Chernobyl power reactor on April 26, 1986 ranks as the world's worst nuclear disaster. The Ukraine blast released many times the radioactive fallout of the U.S. bombing of Hiroshima, Japan in 1945 - as much as 400 times more, according to some radiation experts.

But consensus falls apart over the long-term health effects on people near the epicenter of the blast in Ukraine, Belarus, and Russia. Listen to demographer Murray Feshbach of the Woodrow Wilson International Center for Scholars in Washington.

"Who really knows? We can go [as high as] any number we want and you'll get a source for it," said Murray Feshbach. "Fully we don't know."

A report last year by the Chernobyl Forum - a panel of scientists representing UN agencies and the governments of Ukraine, Belarus, and Russia - says the main health impact has been thyroid cancer cases diagnosed in more than 4,000 children and adolescents as of 2002. Most of these youths have survived, although they must continue to take thyroid medication as long as they live.

As for other cancers, the panel predicts that among the most exposed people - the 600,000 emergency cleanup workers and more than five million residents and evacuees of zones designated as contaminated - total deaths might increase only a few percentage points over what would normally be expected. The Chernobyl Forum estimates those potential excess cancer deaths at 4,000 while a separate World Health Organization study predicts 9,000.

But a spokesman for the Chernobyl Forum, Didier Louvat of the International Atomic Energy Agency, says an accurate forecast is impossible.

"Up to now, we have no reliable data on increased incidence of any somatic disease except thyroid cancer in children and adolescents," said Didier Louvat. "The WHO group spent a lot of time looking at all the data provided from all sources."

Critics are attacking the United Nations data as gross underestimates. The environmental group Greenpeace has released its own study based on Belarus national cancer data predicting 93,000 fatal cancers attributable to Chernobyl. Other anti-nuclear groups produce higher estimates.

The head of Triax Pharmaceuticals, Leonard Mazur, takes issue with the Chernobyl Forum's assertion that most of the cleanup workers and residents of contaminated areas got little more radiation than is normal in the environment. Mazur, who is also a director of the Children of Chernobyl Relief and Development Fund, says low doses can still be harmful.

"It is well known that children and unborn infants are particularly sensitive to the influence of ionizing radiation, even in very small doses," said Leonard Mazur. "The [U.S.] National Academy of Sciences repeatedly states that there is no safe dose of radiation. Even a tiny particle of plutonium, if ingested in a lung, can cause cancer over time."

Mazur and others noted at a Woodrow Wilson International Center discussion that the Chernobyl radioactive cloud was not limited to Ukraine, Belarus, and Russia, but spread over large parts of Europe. Studies measuring the radiation in soil samples have found that outside those three former Soviet republics, the contamination was highest in Greece and Austria and also high in the Scandinavian countries. A 1996 University of Athens study, for example, noted an excess number of leukemia cases in Greek children born after the Chernobyl explosion.

Murray Feshbach wonders why the Chernobyl Forum limited itself to assessing the impact on only the three countries most directly affected by the nuclear blast.

"What about the people from Uzbekistan? What about the people from other areas? The Swiss consider that they have lost people," he said. "The Swedes do, too. Why exclude them?"

Chernobyl Forum spokesman Didier Louvat says the panel's mission was to investigate only the nuclear blast's effect on Ukraine, Belarus, and Russia so that the U.N. Development Program could recommend ways to remedy the effects.

He points out that the report does not discount potential health cost of low amounts of radiation experienced by most of the people in those countries and other regions.

"I never said at low doses there is no effect," he said. "I said there is no consensus of what an effect can be. Regarding plutonium, I agree that the ingestion of any microgram of plutonium can lead to a cancer."

But Louvat says the health effect of low amounts of Chernobyl radiation has been difficult to detect in the research data.